1. Field of the Invention
The present invention relates to image processing apparatuses and control methods thereof, and an image capture apparatus. The present invention particularly relates to techniques for reducing differences in the tints of captured images caused by the optical properties of a lens unit.
2. Description of the Related Art
In digital cameras, images used for recording, display, and the like are generated by performing image processing such as white balance adjustment and color processing on RAW images obtained by an image sensor such as a CCD.
White balance adjustment refers to a process that adjusts the gain of respective red (R), green (G), and blue (B) signal values obtained from the image sensor, whereas color processing refers to a process for adjusting the darkness and color tone of colors, a color conversion process that employs a three-dimensional lookup table (3D-LUT), or the like.
White balance adjustment, color processing, and the like are carried out in order to correctly express the tint of a subject by correcting the color cast, drops in color reproducibility, and so on of a captured image.
Although the color temperature of the light source and the spectral characteristics of the image sensor are the primary causes of changes in the tint of captured images, other factors exist as well. Japanese Patent No. 4337161 proposes setting the white balance based on image shooting conditions, such as a pupil position and an aperture value, in order to reduce color cast in a captured image caused by changes in the angle at which light rays are incident on the microlenses of a CCD that result from the image shooting conditions.
The tint of the captured image is furthermore affected by the spectral transmittance of the imaging lens. With a camera that has interchangeable lenses, there are cases where the spectral transmittance differs depending on the type of interchangeable lens that is attached to the camera.
For example, there are interchangeable lenses that can be attached to multiple types of cameras, such as interchangeable lenses that can be attached to both film cameras and digital cameras. Film cameras are different from digital cameras in that white balance adjustment, color processing, and so on cannot be carried out at will. In order to prevent the tint of captured images from changing greatly depending on the type of interchangeable lens that is attached, the optical members in interchangeable lenses that can be attached to film cameras are designed so that the color characteristics resulting from the spectral transmittance fall within a given constant range.
On the other hand, white balance adjustment, color reproducibility processing, and so on can be carried out at will in digital cameras, and thus interchangeable lenses that can be attached only to digital cameras need not have the same color characteristics resulting from the spectral transmittance as interchangeable lenses that can be attached to film cameras. There are cases where effects such as miniaturization, a reduction in aberration, and so on can be achieved by designing the spectral transmittance with a certain degree of freedom.
An example of the color characteristics of interchangeable lenses is shown in FIG. 2. FIG. 2 illustrates color characteristics of lenses computed for a given light source, based on relative spectral distributions of the light source and the spectral transmittances of the lenses. Here, a lens type 1 indicates an interchangeable lens that can be attached to both a film camera and a digital camera, whereas a lens type 2 indicates an interchangeable lens that can be attached only to a digital camera. Although there are various types of interchangeable lenses that are classified as lens type 1 lenses, the respective color characteristics thereof are distributed within a distribution region central to a point a in FIG. 2. Like the lens type 1, there are various types of interchangeable lenses that are classified as lens type 2 lenses, and the respective color characteristics thereof are distributed within a distribution region central to a point b in FIG. 2.
Because both the lens type 1 and the lens type 2 can be attached to digital cameras, there are digital cameras to which both lens types can be attached. Although it is necessary for the shape of the lens mount to actually fit, there are also cases where a lens can be attached by using a conversion adapter.
However, a camera to which such interchangeable lenses that have different color characteristics can be attached has a problem in that the tint of captured images will differ depending on the lens that is attached. Although the conventional technique disclosed in Japanese Patent No. 4337161 discusses correcting changes in the tint caused by differences in image shooting conditions such as the aperture value, the pupil position, and so on, Japanese Patent No. 4337161 neither discloses nor suggests changes in the tint caused by the spectral transmittance of the imaging lens or a method for correcting such changes.